1. Field of the Invention
The present invention relates to optical instruments utilizing birefringent liquid crystal cells and, more particularly, to apparatus for minimizing beam collimation sensitivity in optical instruments which utilize birefringent liquid crystal cells.
2. Description of the Prior Art
Birefringent cells have the property that their refractive index, hence the velocity of propagation of light therethrough, varies with direction through the cell. Such materials are said to be anisotropic. When a polarized light beam passes through a birefringent cell, the beam breaks into two orthogonally polarized components called the ordinary and extraordinary rays, which propagate with velocities which are inversely proportional to the two refractive indices existing in the directions of beam propagation. These two beams emerge from the birefringent cell with a difference in phase angles. The resultant beam, in general, is said to be elliptically polarized.
When the elliptically-polarized light beam passes through a polarizer, only those components of the beam with their electric vectors in the plane of polarization of the polarizer pass therethrough. The two component beams then interfere and the intensity of the resultant beam is a function of the relative phase angles of the component beams. The greater the product of birefringence and optical path through the birefringent material, the greater the phase angle shift.
If the slow ray emerges with its phase angle 90.degree. behind the fast ray, the two beams totally interfere and the intensity of the beam falls to zero. If the slow ray emerges with its phase angle 180.degree. or some multiple of 180.degree. behind the fast ray, the two rays interfere constructively and there is no dimunition of beam intensity.
Assuming that the optic axis of a birefringent cell has been oriented 45.degree. from the parallel planes of polarization of polarizers on opposite sides thereof, the phase angle difference between the component beams is a function of the difference in refractive indices in the orthogonally propagating beam directions, the birefringence, .DELTA.n, the thickness, d, of the birefringent material, and the wavelength, .lambda., of light. Thus, the relative phase angle is: ##EQU1##
The intensity of light passing through the second polarizer, neglecting any absorption losses, is given by the equation: EQU I=1-sin.sup.2 (.pi..DELTA.nd/.lambda.).
Most birefringent materials have a fixed refractive index, although it is possible to change the birefringence in some materials. For example, this has been accomplished by utilizing stressed polymer films as variable retarders. A variable birefringence can also be induced in almost any liquid by applying a strong electric field. The Kerr cell is such a device. However, none of these materials are very useful because of a variety of complicating factors.
On the other hand, it is a relatively simple matter to vary the birefringence in a liquid crystal cell simply by varying the voltage applied thereto. Because of this characteristic, liquid crystal cells are excellent candidates for use in a variety of different types of optical instruments.
By way of example, there is disclosed, in my copending application, Ser. No. 045,725 now U.S. Pat. No. 4,394,069 filed concurrently herewith and entitled Liquid Crystal Tuned Birefringent Filter, a moderately narrow-band, tunable, birefringent filter using zero-twist, nematic-phase, liquid crystal cells as variable retarders. Such patent application also describes a variety of different types of optical instruments which make use of the variable birefringence characteristics of such a filter.
What is basic in any optical instrument which includes a birefringent liquid crystal cell is that a beam of polarized light is passed through the cell. The effective birefringence of such a cell depends upon the angle between the optic axis thereof and the propagation direction of a ray passing through the cell. The optical path length for a beam passing through the cell is the product of the refractive index of the cell in the plane of beam propagation and path length. Thus, a liquid crystal cell can be used to vary retardation or optical path length. Tuning with the liquid crystal cell is accomplished by tilting the average optic axis of the liquid crystal molecules. It is not essential that all of the liquid crystal molecules have parallel optic axes as long as all of these axes lie in the plane of beam polarization. However, if all of the rays are not parallel (collimated), each ray will suffer a different retardation, not only because of propagation along paths of different lengths, but also because of propagation along paths of different indices of refraction.
If the optic axis of the liquid crystal molecules is normal to the beam propagation direction, the above two factors compensate one another. However, it is impractical, in a liquid crystal cell, to orient the optic axis of the liquid crystal molecules parallel to the cell surface and normal to the light beam. The reasons for this is that upon the application of an electric field to such a cell, the molecules would not know which way to tilt and would exhibit a breakup pattern leading to intensely scattered light. For this reason, it is necessary to align the liquid crystal molecules with a uniform finite surface director tilt with no electric field applied between the plates and, for this reason, optical instruments including liquid crystal cells are more sensitive to beam collimation than are instruments using conventional birefringent elements.
Because of the above, optical instruments utilizing birefringent liquid crystal cells require the light beam to be collimated. When using a lens with a point source, there is little difficulty from beam collimation. However, if the source is large, collimation will be poor. Spatial filters are often used to reduce the apparent size of a source, but at considerable expense in beam power. However, this has been the only practical solution available heretofore.